The invention relates to a control method for an exhaust aftertreatment system.
Present regulatory conditions in the automotive market have led to an increasing demand to improve fuel economy and reduce emissions in present vehicles. These regulatory conditions must be balanced with the demands of a consumer for high performance and quick response for a vehicle.
A diesel engine has an efficiency of up to about 52% and is thus the best converter of fossil energy. NOx emission concentration, i.e. the emission of nitrogen oxides NO and NO2, is dependent upon local oxygen atom concentration and the local temperature. Said high efficiency is however only possible at an elevated combustion temperature at which high NOx levels are inevitable.
Moreover, a suppression of NOx formation by internal means (air/fuel ratio) has the tendency to cause an increase in particulates, known as the NOx-particulates trade off. Furthermore, an excess of oxygen in the exhaust gas from a diesel engine prevents the use of stoichiometric 3-way-catalyst technology for reduction of NOx as is used in gasoline engine cars from the late 80-ties.
Both carbon particulates and NOx are typical emissions in the exhaust gas of diesel engines. Requirements for reducing such emissions increase and trigger various approaches in the art to reduce emissions. In the European patent EP 1 054 722 B1 an exhaust aftertreatment system is disclosed which combines a particulate filter collecting soot and nitrogen-oxides reduction catalysts in the exhaust tract. For removing soot NO2 is generated by oxidation of NO in an oxidation catalyst. Soot which is collected in a particulate filter is oxidized by NO2. Residual amounts of NO and NO2 in the exhaust gas are reduced to nitrogen gas in a selective-catalytic-reduction catalyst (SCR catalyst) by injecting ammonia into the SCR catalyst.
During operation all catalysts degrade due to accumulation of poisons, thermal migration of the catalyst material etc. This degradation seriously influences the operation of aftertreatment systems. Therefore it is desirable to detect the degradation of a catalyst in the aftertreatment system before the operation of the aftertreatment system fails or legal requirements cannot be fulfilled because of the degradation.
It is desirable to provide an improved exhaust aftertreatment system control method. It is also desirable to provide an adequate improved exhaust aftertreatment system.
In a first aspect of the present invention it is provided a method for an exhaust aftertreatment system (10) of an engine (12) comprising at least one selective catalyst reaction (SCR) at least one clean up catalyst downstream said SCR, an urea injector upstream said SCR and a first NOx sensor downstream said clean up catalyst, the method comprising the steps of injecting a predetermined amount of urea, by said injector, providing a second NOx sensor between said SCR and said clean up catalyst, measuring the NOx content received by said first NOx sensor, measuring the NOx content received by said second NOx sensor, comparing said first and said second NOx content with each other, reducing the predetermined amount of urea if said first NOx sensor is measuring a higher NOx content than said second NOx sensor.
An advantage with this embodiment of the present invention is that it gives to possibility to detect ammonia slip and control ammonia/urea injection directly, while at the same time maintaining low NOx.
Another advantage of the present invention is that it significantly reduces the need for continuous calibration of the SCR models. The accuracy of estimated stored NH3 increases when the ammonia slip after the SCR Is either estimated or measured.
Yet another advantage of the present invention is that it compensated for ageing and deterioration of system performance.
Still another advantage of the present invention is that it gives the possibility too detect system malfunctioning/poisoning.
In another example embodiment of the present invention it is further comprising a look up table having a reduced urea amount related to the difference in NOx measured by the first NOx sensor and the second NOx sensor.
An advantage of this embodiment is that the amount of urea can more quickly be adapted and adjusted to the current circumstances compared to a feed back loop which is the other possibility.
In still another example embodiment according to the present invention said urea amount is reduced in predetermined increments until the NOx measured by said first NOx sensor subtracted by the NOx measured by the second NOx sensor is smaller than a predetermined δNOx.
An advantage of this embodiment is that no prior measurements need to be done, the diagnostic method is self calibrating.
In another aspect of the present invention it is provided an exhaust after treatment system comprising at least one selective catalyst reaction (SCR) at least one clean up catalyst downstream said SCR, an urea injector upstream said SCR and a first NOx sensor downstream said clean up catalyst, characterized by a second NOx sensor provided between said SCR and said clean up catalyst.
In yet another example embodiment of the present invention it further comprising a control unit for comparing the detected NOx values from said first and second NOx sensors, wherein said control unit further comprising means for controlling the amount of urea injected by said urea injector where said amount of urea is dependent on the NOx values from said first and second NOx sensors.
In the drawings, equal or similar elements are referred to by equal reference numerals. The drawings are merely schematic representations, not intended to portray specific parameters of the invention. Moreover, the drawings are intended to depict only typical embodiments of the invention and therefore should not be considered as limiting the scope of the invention.